Numerous polymer systems that contain free carboxy groups are known in the art. It is difficult, however, to prepare a carboxy polyurethane, that is, a polyurethane having free carboxyl groups, because isocyanate, which is a necessary component in the preparation of any polyurethane, is quite reactive with the carboxyl groups of the carboxylic acid reactants used to introduce the carboxyl group.
One approach to the introduction of carboxy groups into a polyurethane resin chain is described in U.S. Pat. No. 3,412,054 to Milligan et al. In that patent, a 2,2-di(hydroxymethyl)alkanoic acid is reacted with an organic diisocyanate to produce a polyurethane containing unreacted carboxylic acid groups. These acids are unique because their carboxyl groups do not react to any significant extent with the isocyanates to prevent the formation of the desired carboxy resin. However, very few carboxylic acids have this character, thus reducing the cost effectiveness of this approach.
Another approach is that of U.S. Pat. Nos. 4,156,066 and 4,156,067 to Gould. In these patents, a polyfunctional lactone, preferably containing at least three hydroxyl groups, is reacted with an isocyanate and one or more diols to form a polyurethane having lactone groups in the polymer backbone. Upon saponification or hydrolysis the lactone rings open up to form carboxyl groups. However, the amount of carboxyl which can be introduced via the lactones is limited such that the enhancement of properties attributable to the carboxyl groups, e.g., water-solubility, cross-linkability or other reactivity characteristic of carboxyl functionality, is marginal for some applications.